1. Field of the invention
This invention relates to an adaptive echo canceller for cancelling echo signals which inherently occur in long distance 2-wire communications networks, and more specifically to such a canceller which prevents tap coefficients of a transversal filter from drifting toward excessively large values or from divergence.
2. Description of the Prior Art
2-wire communications networks have been constantly plagued by "echo". A known approach to solving this problem is the use of echo suppressors. An echo suppressor opens a send-path to block echo when it detects signal on a receive-path. However, the decision to open is overruled if the return signal from a 4-wire to 2-wire hybrid circuit is deduced to contain near-end party's signal irrespective of the presence of echo to permit the near-end party's signal to be transmitted to the far end. The echo suppressor is thus inherently incapable of blocking echo when both parties are trying to talk simultaneously, i.e., during so-called double talk. Another problem encountered with this type of arrangement is that it imparts a chopping to speech by the opening and closing of the transmission path. Both of these shortcomings have been found more annoying particularly with the echo encountered with satellite circuits.
In order to overcome these problems, adaptive echo cancellers have been proposed. An adaptive echo canceller actually estimates the echo and then subtracts it out of the received signal, and hence is capable of much better performance due to the absence of the opening and closing of the transmission line.
An adaptive echo canceller includes an echo estimator in the form of a transversal filter. The characteristics of the echo estimator are determined using incoming signals. The stable operations of the adaptive echo canceller requires that the incoming signals have a frequency bandwidth or spectrum equal to or wider than that of the actual echo path. More specifically, the echo cancellation is limited to the frequency bandwidth of the incoming signals. No problem therefore exists where the incoming signals are signals having a wide frequency band such as voice signals or white noises in that the incoming signal is capable of covering the actual echo path in terms of frequency bandwidth.
However, there are some cases where signals having relatively a narrow frequency band, such as test tone signals or MODEM (Modulator-Demodulator) signals, are successively applied to an adaptive echo canceller. In such a case, the adaptive echo canceller fails to estimate echo with respect to the frequency spectrum not involved in the incoming signals. When the estimation is disturbed by external noises entering the echo signals, arithmetic errors are generated. These errors do not adversely affect the estimation process of the echo signals if the frequency spectrum of the incoming signals is equal to or extends over the frequency spectrum of the actual echo path.
However, in the case where the incoming signals have a narrow frequency bandwidth, it is no longer expected to exactly estimate a transfer function for identifying the characteristics of the echo estimator. If such identification continues for a long period, arithmetic errors are accumulated to result in the possibility of the dynamic range of a tap coefficient memory being exceeded. Under these divergence conditions, it is impossible to cancel the echo signal.
In order to overcome these problems, Donald L. Duttweiler disclosed in U.S. Pat. No. 4,243,959 an echo canceller entiled "Adaptive Filter With Tap Coefficient Leakage". However, the adaptive filter in this prior art patent complex requires a circuit arrangement.